Slotted waveguide antenna array



Oct. 25, 1955 s. HERSHFIELD ET AL 2,721,939

SLOTTED WAVEGUIDE ANTENNA ARRAY Filed May 1, 1951 INVENTORS SANFORDHERSHFIELD WILLIAM J. FOLEY ATTORNEY United States Patent Ofiice2,721,939 Patented Oct. 25, 1955 SLOTTED WAVEGUIDE ANTENNA ARRAY SanfordHersllfield and William V. Foley, Middle River,

Md., assignors to The Glenn L. Martin Company, Middle River, Md., acorporation of Maryland Application May 1, 1951, Serial No. 223,974

3 Claims. (Cl. 250-33) This invention relates to an improved slottedwaveguide antenna array, particularly adapted for use where it isnecessary to limit the lateral width of the array.

It is frequently desirable, especially in military aircraft, to locatean antenna within the confines of the fin or rudder thereof. Where ahigh-speed aircraft is involved, the lateral thickness of the fin orrudder is extremely limited and, as a result, if an antenna is to beconstructed therein, it must likewise be of relatively narrowdimensions.

It is an object of this invention to provide a slotted waveguide antennaarray wherein the slots are located in the narrow wall of a rectangularwaveguide section thereby producing an antenna which meets the aboverequirement.

Another object of this invention is to provide an antenna array ofrelatively narrow width which will have a relatively broad pattern in aplane perpendicular to the axis of the wave guide, while at the sametime having a narrow radiation pattern in a plane passing through theaxis of the wave guide.

It is a further object to provide a slotted wave guide antenna arraywherein the slots are in the narrow face of the wave guide and yet maybe excited in the proper in-phase relationship so as to give the desiredpattern.

Further and other objects will appear from the following description andclaims when taken in view of the accompanying drawings.

In the drawing:

Figure 1 is a fragmentary perspective view of the complete antenna unit.

Figure 2 is a transverse sectional view through the antenna.

The antenna array 1 comprises an elongated section of rectangularwaveguide 2 having a plurality of radiating slots 3 formed in one of thenarrow walls 4 thereof. The particular antenna shown has five suchslots, although to better show the construction, only three of themappear in the drawing. The number of slots will of course depend uponthe radiation patterns desired in any particular case. As shown inFigure 1 the slots are arranged along the center line of the narrow walland each has an effective length equal to one-half of the wave length inspace at the design frequency. The centerrtorcenter spacing of theslots'is equal to a full wa e lfl th in space. Secured within thewaveguide adjacent its upper end is a non-contacting shorting element 5which forms an eifective short circuit across the waveguide 2 in theplane of its lower face 6. This face of the shorting member is locatedthree-quarters of a wave length above the center of the uppermost slot3.

Spaced one-half wave length below the center of the lowest slot 3 are apair of metal blocks 7 firmly secured to the broad walls 8 of thewaveguide as at 9. As is clear from the drawing, blocks 7 are locatedalong the median plane of the waveguide and together with the walls 4and 8 of the guide, form a ridge waveguide section which serves as animpedance matching transformer 2 section between the radiating portionof the array and a coaxial feed line 10.

In order to symmetrically excite the waveguide from the un-symmetricalconductors of the coaxial cable, .opposed portions of the outerconductor 11 are cut away to provide a pair of slits 12, each having alength equal to one-quarter wave length at the design frequency, whichslits divide the end portion of the outer conductor into two symmetricalhalves, each half being electrically connected to the adjacent block 7.The center conductor 13 is then bridged across to the junction ofone-half of the outer conductor with its associated block as indicatedat 14. This forms what is known in the art as a balun connection. Thecoaxial line 10 extends downwardly from the blocks 7 through the metalend wall 15 of the impedance matching section and then laterallyoutwardly through one of the walls of the waveguide to a suitablecoaxial connector 16.

As previously set forth, the slots 3 have a center-tocenter spacingequal to one wave length in space. This spacing is essential in order toobtain the desired narrow beam width in the vertical plane without theproduction of undesired side lobes. However, it is a well knowncharacteristic of air-filled waveguide that the apparent wave lengthwithin the guide is greater than that in free space. Yet, to excite theslots 3 in the desired in-phase relationship, the fields within thewaveguide must be the same at each of the slots. In order to overcomethis difiiculty, applicants have partially filled the wave guide with anelongated block 17 of suitable dielectric material of such dimensions asto make the effective wave length within the guide equal to that in freespace. In the particular antenna shown, the dielectric material consistsof a block of polystyrene which extends between the shorting member 5and the element 15 along the side of the waveguide opposite from theslots 3. The thickness of the block required to bring about the desiredcorrection in wave length will depend upon the dimensions of the guide,the frequency for which the array is designed, the material used and itsrelative location within the guide. In the instant case, usingpolystyrene and locating it as shown in the drawing, the properthickness was determined to be approximately one-third of the widerdimension of the guide.

The operation of the antenna is believed to be apparent from the abovedescription. It may, of course, be used with equal effectiveness eitherfor transmitting or receiving. Assuming that the antenna is to be usedfor transmitti l radio energy at the desired frequency will be conductedto the antenna by means of the coaxial cable 10 and will excite thewaveguide so as to produce standing waves therein. The matchingtransformer formed by the blocks 7 and end wall 15 will serve to matchthe impedance of the transmission line 10 to that of the wave guide 2 soas to prevent standing waves occurring on the line 10. Due to thepresence of the dielectric block 17 within the waveguide, the apparentwave length of the standing waves within the waveguide, when measuredalong the slotted wall, will be exactly equal to that in free space andwill, as a result, cause the excitation of the radiating slots 3 in thedesired in-phase relationship. The radiation pattern in the verticalplane is relatively narrow while that in the horizontal plane is quitebroad. Field strength measurements of a 5-slot array, constructed asdescribed above, showed a half power beam width of only about 1l in thevertical plane, and of about in the horizontal plane.

Since the radiation is from the narrow wall of the guide, the antennamay be located within a thin airfoil or control surface so that itsmajor axis of propagation is in the plane of the airfoil as isfrequently required.

Obviously, as is well understood in the art, the dimensions of thematching transformer section 7 will depend upon the number of slots andthe resulting impedance of the array as well as upon the type of feedline employed. While but one form of the invention has been shown, manychanges can be made without departing from the basic concept of theinvention. Thus, dielectric block 17 could be made of other materialsthan polystyrene and could be located in other portions of the waveguideso long as it is designed to bring about the desired correction of wavelength therein. The dielectric block need extend only throughout theslotted portion of the waveguide since the apparent wave length in theother portions of the guide will not adversely affect the radiationcharacteristics of the antenna so long as the slots are excited in thedesired in-phase relationship. Other known types of feed could obviouslyalso be provided without affecting the operation.

We claim as our invention:

1. An antenna array for electromagnetic waves of predetermined frequencycomprising an elongated rectangular tube having one internal dimensionof said tube substantially narrower than the other dimension of therectangle, one of the narrower Walls of said tube having formed thereina plurality of elongated collinear slots spaced longitudinally alongsaid wall, the center-to-center spacing of said slots beingsubstantially equal to a wavelength in space at said frequency, anassembly adapted to be placed within said tube consisting of a strip ofdielectric material of a width substantially equal to the narrowerdimension of said tube, a metal end wall transverse of said tube, anon-contacting shorting element spaced from said end wall in said tubeby the longitudinal extent of said dielectric strip, the face of saidshorting element being located approximately three-quarters wavelengthfrom the center of the nearest of said slots, an impedance matchingtransformer positioned between the end slot and said metal end wall,said transformer being positioned approximately one-half wavelength fromthe center of the nearest slot, a coaxial feed line extending throughsaid end wall to a point adjacent said transformer, means providing aconnection between the said coaxial feed line and the said transformersuch that electromagnetic energy applied thereto in an unbalanced linesense is transformed to a balanced line sense, said dielectric stripmaterial extending between said end wall and said shorting element andpartially filling said tube, whereby the apparent wavelength of saidstanding waves as measured longitudinally along said tube at the narrowedge thereof coincides with that of the wavelength in space at saidfrequency.

2. An antenna array for electromagnetic waves of predetermined frequencycomprising an elongated rectangular tube having one internal dimensionof said tube substantially narrower than the other dimension of therectangle, one of the narrower walls of said tube having formed thereina plurality of elongated collinear slots spaced longitudinally alongsaid wall, the center-to-center spacing of said slots beingsubstantially equal to a wavelength in space at said frequency, a metalend wall extending transverse of one end of said tube, a non-contactingshorting element at the other end of said tube, the face of saidshorting element being located approximately three-quarters wavelengthfrom the center of the nearest of said slots, an impedance matchingtransformer positioned between an end slot and said metal end Wall, saidtransformer being positioned approximately one-half wavelength from thecenter of the nearest slot, a coaxial feed line extending through saidend wall to a point adjacent said transformer, means providing aconnection between the said coaxial feed line and the said transformersuch that electromagnetic energy applied thereto in an unbalanced senseis transformed to a balanced line sense, including a strip of dielectricmaterial extending between said end wall and said shorting element andpartially filling said tube, whereby the apparent wavelength of saidstanding waves as measured longitudinally along said tube at the narrowedge thereof coincides with that of the wavelength in space at saidfrequency.

3. An antenna array for electromagnetic waves of predetermined frequencycomprising an elongated rectangular tube having one internal dimensionof said tube substantially narrower than the other dimension of therectangle, one of the narrower walls of said tube having formed thereina plurality of elongated collinear slots spaced longitudinally alongsaid wall, the center-to-center spacing of said slots beingsubstantially equal to a wavelength in space at said frequency, anassembly adapted to be placed within said tube consisting of alongitudinal spacer strip of dielectric material of a widthsubstantially equal to the narrower dimension of said tube, a metal endwall transverse of said tube at one end of said strip, and anon-contacting shorting element in said tube at the other end of saiddielectric strip, the face of said shorting element being locatedapproximately three-quarters wavelength from the center of the nearestof said slots, an impedance matching transformer positioned between theend slot and said metal end wall, said transformer being positionedapproximately one-half wavelength from the center of the nearest slot, acoaxial feed line extending through said end wall to a point adjacentsaid transformer, means joining the said coaxial feed line to the saidtransformer such that electromagnetic energy applied thereto in anunbalanced line sense is transformed to a balanced line sense, saiddielectric strip material partially filling said tube, whereby theapparent wavelength of said standing waves as measured longitudinallyalong said tube at the narrow edge thereof coincides with that of thewavelength in space at said frequency.

References Cited in the file of this patent UNITED STATES PATENTS2,405,242 Southworth Aug. 6, 1946 2,433,368 Johnson et al Dec. 30, 19472,461,005 Southworth Feb. 8, 1949 2,479,209 Chu Aug. 16, 1949 2,482,162Feldman Sept. 30, 1949 2,573,461 Lindenblad Oct. 30, 1951 2,573,746Watson et al Nov. 6, 1951 2,574,433 Clapp Nov. 6, 1951 2,605,413 AlvarezJuly 29, 1952 OTHER REFERENCES Jordan & Miller Slotted Cylinder Antenna,in Electronics, February 1947, pages to 93.

